Mechanism of methane flow through sheared coals and its role on methane recovery

Tectonically sheared coals are closely related with coalbed methane exploitation, and are classified into cataclastic coals and mylonitic coals in view of their deformation mechanism, either brittle or ductile. Scanning electron microscopy and reflected light microscopy observations showed that cataclastic coals possess a hierarchy of open, continuous and connecting fractures, whereas mylonitic coals always display tightly compressed and collapsed fractures. Mercury porosimetry studies also indicated that compared with normal coals, cataclastic coals possess greater porosity, more specific surface area and wider average fracture aperture; whereas mylonitic coals are characterized by narrower average fracture aperture and a great deal of specific surface area. Based on experimental data and actual site experiences, a new model of methane flow within coals was proposed. Gas flow through sheared coals, unlike normal coals—a simple dual porosity system comprising a matrix of micropores that are surrounded by cleats, also contain additional steps because of overwhelming sheared fractures and different deformation mechanisms. This model can explain why gas outbursts are always accompanied by small-scale compressively geological structures where mylonitic coals often occur, and why in such area the rate of gas extraction is unexpectedly lower, despite the presence of a great volume of methane.